DBR lasers having a special grating such as an SG (Sampled Grating) or SSG (Super Structure Grating) have been developed for use as wavelength tunable semiconductor lasers to provide any wavelength within the C-band used in wavelength division multiplexer (WDM) systems. In order for these DBR lasers to oscillate at a desired wavelength, it is necessary to control currents to adjust not only the wavelength but also the phase of the light, resulting in a complicated control system. Furthermore, it has been difficult to ensure the long-term reliability of the lasers in terms of wavelength. To overcome the above problems, short cavity DBR lasers have been recently developed which do not require phase control of the light. See, for example, Selected Topics in Quantum Electronics, IEEE Journal, vol. 9, September to October 2003, pp. 1132-1137 (Nonpatent Document 1). However, the wavelength of each channel of these short cavity DBR lasers can be varied only by 10 nm or less. (This range over which the wavelength can be varied is referred to as a “wavelength tunable range”.) To increase the wavelength tunable range so as to cover the entire C-band, it is necessary to array a plurality of DBR lasers. In one known semiconductor laser device, for example, the light waves from an array of DBR lasers are combined by a multi-mode interferometer (MMI) and amplified by a semiconductor optical amplifier (SOA).
FIG. 1 shows a laser chip on which a 4 channel DBR laser array, an MMI, and an SOA are monolithically integrated. This example has basically the same geometric configuration as the semiconductor laser devices of the present invention. It should be noted that the wavelength tunable semiconductor lasers of the present invention are different from conventional wavelength tunable semiconductor lasers in dimensional design, as described later in detail. In the laser chip, gain electrodes 101, 102, 103, and 104, DBR electrodes 105, 106, 107, and 108, and an SOA electrode 114 are formed on the surface of an InP substrate 100. It should be noted that in the figure, reference numerals for these electrodes (such as “101”) are written beside their electrode pad portions. In the top and cross-sectional views (in FIGS. 2A to 2C described later), the portions of these electrodes directly formed in the waveguide portions, etc. are referred to as “rear DBR electrode 161”, “gain electrode 162”, “front DBR electrode 163”, “SOA electrode 164”, etc., respectively. These portions are connected to their respective electrode pads, as shown in the top view. It should be noted that in these figures, the portion of each DBR electrode on the light emitting side (or combiner side) is referred to as the “front DBR electrode”, while the portion on the opposite side (or input side) is referred to as the “rear DBR electrode”.
The waveguide structure is such that DBR laser channels ch1, ch2, ch3, and ch4 (or 115, 116, 117, and 118) extend in parallel to one another and are connected at the light emitting side to optical waveguides 109, 110, 111, and 112, respectively. The optical waveguides 109 to 112 are connected to an MMI combiner 113 and further connected to the SOA waveguide under the SOA electrode 114. FIG. 2A is a top view of the chip shown in FIG. 1; and FIG. 2B is a cross-sectional view of the chip shown in FIG. 2A taken along line A-B-C-D-E. Specifically, FIG. 2B shows a cross section taken along an optical path that passes through the semiconductor laser portion (from point A to point B) shown in the upper left of the figure, an optical waveguide (B-C), the optical combiner (C-D) for combining a plurality of optical waveguides, and the semiconductor optical amplifier (D-E).
The laser chip has integrated therein a rear DBR region. 138, a gain region 139, a front DBR region 140, an S-shaped waveguide region 141, an MMI region 142, and an SOA region 143. The length of the gain region is reduced to 15 μm to increase the range over which the wavelength can be continuously varied (referred to as the “continuous wavelength tunable range”). Further, front and rear end faces 144 and 145 have a low reflective film coating thereon. The DBR laser portion includes a core layer 132 of the gain region, a refractive index control core layer 134 of the rear DBR region, a refractive index control core layer 135 of the front DBR region, and a grating supply layer 136. The refractive index control core layers 134 and 135 are connected to the core layer 132, and the grating supply layer 136 is formed on the refractive index control core layers 134 and 135. The refractive index control core layer 135 of the front DBR region is also connected to one end of a core layer 137 of the low-loss optical waveguide including the S-shaped waveguide region 141 and the MMI region 142. Further, a core layer 133 of the SOA region is connected to the other end of the core layer 137.
FIG. 3 shows exemplary wavelength characteristics of the laser chip shown in FIG. 1. The four DBR laser channels ch1, ch2, ch3, and ch4 handle different wavelength regions, thereby collectively covering the entire C-band. For example, to generate 1540 nm wavelength light, currents are injected to the gain layer and the SOA layer of the channel ch2 and, at the same time, a current of 50 mA is injected to the DBR layer of the channel ch2. No current is injected to the gain layers and DBR layers of the other DBR laser channels.
[Nonpatent Document]
Selected Topics in Quantum Electronics, IEEE Journal, vol. 9, September to October 2003, pp. 1132-1137.
In order to be able to generate all the wavelengths in a desired range (e.g., the C-band), the above laser chip must be configured such that the channels (ch1 to ch4) are seamlessly connected to one another in terms of wavelength. That is, the DBR current values of these channels at which mode hopping occurs must be appropriately adjusted relative to one another. FIG. 3 above is a diagram showing the relationship between the DBR current and the oscillation wavelength (of each channel). More specifically, the figure shows the wavelength characteristics of the channels ch1, ch2, ch3, and ch4 that cover different wavelength regions. As shown in the figure, there is a jump in the wavelength characteristic curve of each channel (i.e., mode hopping) at approximately 10 mA.
To appropriately adjust the mode-hopping DBR current values of the channels relative to one another, the refractive index of each DBR layer must be set with an accuracy of ±0.002 or better and a distance Lgrt (described below) must set with an accuracy of ±0.02 μm. The distance Lgrt refers to the distance between the center portion of the one of rear grating elements closest to the gain region 139 and the center portion of the one of front grating elements closest to the gain region (wherein the rear grating elements and the front grating elements are formed in the rear DBR region and the front DBR region, respectively, and make up the grating 136). Specifically, the thickness of each DBR layer must be set with an accuracy of ±6 nm and the composition wavelength of each DBR layer must be set with an accuracy of ±10 nm, requiring highly accurate crystal growth and process techniques.
Another problem is that increasing the continuous wavelength tunable range requires the length Lgain of the gain region 139 to be reduced, making it difficult to achieve laser oscillation. FIG. 4 shows calculated values indicating the dependence of the continuous wavelength tunable range Δλcon on the distance Lgrt. It should be noted that in this example, the grating 136 is divided into two grating portions (made up of rear grating elements and front grating elements, respectively) formed within the rear DBR region 138 and the front DBR region 140, respectively, and each grating portion is spaced 5 μm apart from the gain region 139. (These grating portions sandwich the gain region 139.) The distance Lgrt is related to the length Lgain by the following equation.Lgrt=Lgain+10 μm  (1)It should be noted that the above continuous wavelength tunable range curve shown in FIG. 4 was obtained by assuming that the optical coupling coefficient κ of the grating 139 is 90 cm−1. Thus, FIG. 4 shows an exemplary relationship between the distance Lgrt and the range over which the oscillation wavelength can be continuously varied. As be seen from FIG. 4, the distance Lgrt must be set to less than 25 μm (that is, the length Lgain must be set to less than 15 μm) to achieve a continuous wavelength tunable range of more than 10 nm. This requirement is very difficult to meet if laser oscillation is to be produced. Therefore, practically, such a structure is not suitable for mass production.